The invention relates to a gas-storage power plant having the features of the preamble of claim 1. The invention also relates to a method of throttling a storage pressure to a working pressure for a turbogroup of such a gas-storage power plant.
Such a gas-storage power plant is normally interconnected in a xe2x80x9cCompressed-Air-Energy-Storage Systemxe2x80x9d, in short a CAES system. The basic idea of a CAES system is seen in the fact that excess energy which is generated by permanently operated conventional power generating plants during the base-load times is transferred to the peak-load times by bringing gas-storage power plants onto load in order to thereby use up less resources overall for producing electrical energy. This is achieved by air or another gas being pumped under a relatively high pressure into a reservoir by means of the inexpensive excess energy, from which reservoir the air or gas can be extracted when required for generating relatively expensive current. This means that the energy is stored in a retrievable manner in the form of potential energy. Worked-out coal or salt mines, for example, serve as reservoirs. Since the gas stored in the gas reservoir is normally air, such a gasstorage power plant is generally also designated as an air-storage power plant.
Gas-storage power plants of this type are known, for example, from the report xe2x80x9cCAES REDUCED TO PRACTICExe2x80x9d by John Daly, R. M. Loughlin from Dresser-Rand, Mario DeCorso, David Moen from Power Tech Associates Inc., and Lee Davis from Alabama Electric Cooperative Inc., which has been presented at the xe2x80x9cASME TURBO EXPO 2001xe2x80x9d. Accordingly, a gas-storage power plant normally comprises a gas reservoir, in which a gas can be stored under pressure, and a turbogroup which has at least one turbine. A gas-supply line connects the gas reservoir to the turbogroup, so that the turbine of the turbogroup can be driven with the gas from the gas reservoir. Arranged in the gas-supply line is a control valve arrangement which throttles a storage pressure applied on the inlet side to a working pressure required by the turbogroup. The control valve arrangement normally comprises a control valve, the throttling effect of which can be controlled in an open-loop and/or closed-loop system, and a stop valve which is connected upstream of the control valve and can be switched over between an open position for normal operation and an emergency-trip position.
In this case, the control valve must be actuated in such a way that the required working pressure is always provided on the outlet side. However, this working pressure is variable and depends on the current operating state, or on an operating state to be set, of the turbogroup. In addition, the storage pressure applied on the inlet side may also vary within a relatively large range, since the gas reservoir empties during operation of the turbogroup, a factor which is accompanied by a decrease in the storage pressure. For example, the storage pressure varies within a large range over the course of the operating week by virtue of the fact that the storage pressure is highest at the start of the week and, after daily decrease and partial refilling of the storage cavern at night during the working week, the storage pressure toward the end of the working week is finally at the lowest level. The storage pressure rises again to the level at the start of the week by filling the storage cavern during the weekend. Accordingly, the control range, for example, over the course of a week is very large and the outlay in terms of open-loop or closed-loop control at the control valve for the provision of the currently desired working pressure is relatively high. However, increased outlay in terms of closed-loop or open-loop control at the control valve may be at the expense of the reliability and thus of the operating safety of the control valve arrangement. Furthermore, in particular during start-up and during the loading of the turbogroup, very high pressure differences may be present between the storage pressure applied at the inlet side of the control valve and the working pressure to be provided on the outlet side, for example if the gas reservoir is still filled to the maximum level and the turbogroup only requires a minimum working pressure. Accordingly, the control valve must realize extremely pronounced throttling effects. In the process, vibrations and oscillation excitations, in particular in combination with noise emissions, may occur in the control valve, as a result of which the control valve is highly loaded. High loads may lead to material fatigue and to failure. A control valve which fails during start-up of the turbines may lead to damage or destruction of the turbines.
The invention is intended to provide a remedy here. The object of the present invention is to show a gas-storage power plant of the type mentioned at the beginning which ensures relatively high reliability and operating safety for the valve arrangement.
This problem is solved by the subject matter of the independent claims. Advantageous embodiments are the subject matter of the dependent claims.
The present invention is based on the general idea of designing the throttling of the storage pressure to the working pressure in at least two stages. This measure leads to considerably smaller pressure differences in the individual throttling stages. Accordingly, the loading on the throttling points is reduced. Furthermore, the invention provides considerable simplification of the closed-loop or open-loop control of the individual throttling points by the first throttling stage being controlled in a closed-loop or open-loop system according to the changes in the storage pressure, whereas the last throttling stage is controlled in a closed-loop system according to the changes in the working pressure or the intermediate pressure. Both the reduction in the outlay for the closed-loop or open-loop control of the throttling points and the reduction in the pressure difference occurring at the individual throttling points result in an increase in the reliability and the operating safety of the valve arrangement.
In a preferred embodiment of the invention, the valve arrangement has two throttling stages. There, the first throttling stage is designed as a first control valve, this control valve in particular having the function of an emergency-trip or stop valve and a control function.
The first throttling stage serves to reduce the storage pressure applied on the inlet side to an intermediate pressure. The result of this measure is that the aforesaid intermediate pressure is admitted on the inlet side to the second throttling point, a second control valve, as a result of which the throttling to the working pressure is considerably simplified.
The valve arrangement may of course have any desired number of throttling stages, by means of which throttling is effected to several intermediate pressures and, by the last stage, to the working pressure.
In a further preferred embodiment of the invention, the control valves each have an actuator, these actuators being connected to a control device which is interconnected in a feedback control circuit or a plurality of feedback control circuits. A feedback control circuit for the first control valve directs values of the storage pressure and/or of the intermediate pressure to the control device. A feedback control circuit for a further control valve or the last control valve directs values of the intermediate pressure and/or of operating variables of the turbogroup to the control device.
In a further embodiment, a control valve or a plurality of control valves are designed as relieved single-seat valves. All the control valves are preferably designed as relieved single-seat valves, which permits a cost reduction. A relieved single-seat valve has the advantage that it requires small actuating forces and thus small drives. Small drives also have short actuating times. (Such short actuating times ensure that the overspeed of the air turbine in the event of a blackout remains within the required limits.)
In a further embodiment, the valve arrangement has a bypass valve, which is required when the plant is started up. By means of the bypass valve, the flow bypasses the air turbine, and the requisite air is injected directly upstream of the combustion chamber. With the bypass-valve arrangement, an additional stop member can be saved. The first control or emergency-trip valve is regarded as a redundant valve for the air control valve of the air turbine and the bypass. In a method as claimed in claim 7 for throttling the storage pressure in such a gasstorage power plant, the first control valve of the valve arrangement is controlled in a closed-loop system in such a way that an essentially constant intermediate pressure is generated.
This method is to be used in particular when closed-loop control is to be effected according to the storage pressure varying over the course of the week. In a special method for this purpose, the intermediate pressure is controlled in a closed-loop system by a comparison between a predetermined setpoint and an actual value of the intermediate pressure and by determining the pressure deviation. This closed-loop control is realized by a simple feedback control circuit with the intermediate pressure as command variable.
In a corresponding manner, the second or last throttling stage of the valve arrangement can be actuated as a function of a setpoint/actual-value deviation from a command variable of the turbogroup. The working pressure then occurs according to this command variable. This closed-loop control is realized by a feedback control circuit of simple construction with a corresponding command variable of the turbogroup.
The intermediate pressure which can be set with the first throttling point is expediently selected in such a way that it is greater than or equal to the maximum working pressure. This ensures that the intermediate pressure, during operation of the turbogroup, can be kept essentially constant or within a certain range at all the storage pressures admissible for this and at all the working pressures admissible for this. As a result, the closed-loop and/or open-loop control of the throttling points is simplified at all the pressure differences which occur between storage pressure and working pressure.
In a further method as claimed in claim 9 for throttling the storage pressure of the gas-storage power plant according to the invention, the first control valve is controlled in an open-loop system according to a fixed or predetermined valve position, whereupon throttling is effected to an intermediate pressure which is variable in accordance with the varying storage pressure. The valve position is predetermined according to a storage pressure selected for a certain weekday. However, since the storage pressure can deviate from the selected day pressure, the generated intermediate pressure is correspondingly variable.